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Lakes and reservoirs become stratified in the summer time due to increased solar radiation. This stratification suppresses vertical mixing and transport of heat, oxygen, nutrients, and other dissolved substances and thus acts as a key control on water quality in such aquatic systems. In this seminar, Dr. Danielle Wain from the Department of Architecture & Civil Engineering will present the impact of two sources of mixing in lakes and reservoirs, one natural and one engineered.

Recent advances in oceanography have indicated that vertical migration of zooplankton may contribute as much mixing in the ocean as winds and tides. Many types of zooplankton (in both lakes and oceans) spend the day in darker deeper regions of the water column to hide from predators. At night time, they rise to the surface to feed, possibly transporting nutrient-rich water to the surface by dragging it with them as they swim or by generating a turbulent wake which mixes the water. In this research, we show the first measurements of turbulence and mixing during zooplankton migrations in a lake and discuss the implications for water quality.

At the other end of the spectrum, to combat the negative water quality impacts of summer stratification, water utilities use engineered methods to completely mix entire drinking water reservoirs to minimize the anoxic conditions that lead to release of manganese and iron from the sediments and create ideal conditions for nuisance algae species; both metals and algae can lead to color, taste, and odor problems which are expensive to treat and can lead to low customer satisfaction and fines. Recently, surface mixer systems, which user impellers to nominally pull surface water in and push it to the bottom to destratify and aerate the reservoir, have been gaining popularity, but very little is known about their actual in situ functionality. In this research, we show the first measurements of turbulence and mixing generated by a surface mixer system in small reservoir, how this impacts sediment/water column exchange, and the effect on drinking water supplies.